1. Field of the Invention
The present invention relates to a fuel cell stack formed by connecting a plurality of fuel cell modules together. Each of the fuel cell modules includes units of fuel cells, i.e., the fuel cell module is formed by stacking a plurality of membrane electrode assemblies and separators alternately. The Each of the membrane electrode assemblies includes an anode, a cathode, and an electrolyte membrane interposed between the anode and the cathode.
2. Description of the Related Art
For example, a solid polymer electrolyte fuel cell employs a membrane electrode assembly (MEA) which includes two electrodes (anode and cathode) and an electrolyte membrane interposed between the electrodes. The electrolyte membrane is a polymer ion exchange membrane (proton exchange membrane). Each of the electrodes comprises a catalyst and a porous carbon sheet. The membrane electrode assembly is interposed between separators (bipolar plates). The membrane electrode assembly and the separators make up a unit of the fuel cell (unit cell) for generating electricity. A plurality of unit cells are connected together to form a fuel cell stack.
In the fuel cell stack, a fuel gas such as a hydrogen-containing gas is supplied to the anode. The catalyst of the anode induces a chemical reaction of the fuel gas to split the hydrogen molecule into hydrogen ions (protons) and electrons. The hydrogen ions move toward the cathode through the electrolyte, and the electrons flow through an external circuit to the cathode, creating a DC electric current. An oxygen-containing gas or air is supplied to the cathode. At the cathode, the hydrogen ions from the anode combine with the electrons and oxygen to produce water.
The fuel cell stack is mounted in a vehicle, for example. In the vehicle application, a relatively high power output is required. Therefore, at the time of starting the operation of the fuel cell stack, the fuel cell stack needs to be heated swiftly so that the fuel cell stack output the high power desirably.
For example, a fuel cell device is disclosed in U.S. Pat. No. 6,294,278 B1. As shown in FIG. 8, the fuel cell device has a high temperature stack 1 and a low temperature stack 2. A plurality of fuel cells 3a are stacked together to form the high temperature stack 1. A plurality of fuel cells 3b are stacked together to form the low temperature stack 2. The fuel cells 3a of the high temperature stack 1 and the fuel cells 3b of the low temperature stack 2 are operated at different temperatures.
The inlet side of the high temperature stack 1 is connected to a fuel gas supply passage 4 for supplying a fuel gas (reactant gas) to a fuel gas passage (not shown) in the high temperature stack 1, and an oxygen-containing gas supply passage 5 for supplying an oxygen-containing gas (reactant gas) to a oxygen-containing gas passage (not shown) in the high temperature stack 1. Further, the outlet side of the high temperature stack 1 is connected to connection lines 6a, 6b. The reactant gases are discharged from the outlet side of the high temperature stack 1 into the connection lines 6a, 6b. The connection lines 6a, 6b are connected to a fuel gas passage (not shown) and an oxygen-containing gas passage (not shown) in the low temperature stack 2 through a cooler 7.
The high temperature stack 1 and the low temperature stack 2 are cooled by a cooling air flowing in a direction indicated by an arrow H which is perpendicular to a direction indicated by an arrow L. The cooling air firstly cools the low temperature stack 1, and then, cools the high temperature stack 2.
In the conventional fuel cell device, the connection lines 6a, 6b for supplying the fuel gas and the oxygen-containing gas from the high temperature stack 1 to the low temperature stack 2, and the cooler 7 for cooling the fuel gas and the oxygen-containing gas to the desirable operating temperature of the low temperature stack 2 are required. Therefore, the fuel cell device is not simple, and economical.
In the high temperature stack 1 and the low temperature stack 2, the fuel cells 3a, 3b are stacked in the direction indicated by the arrow L, and the cooling air flows in the direction indicated by the arrow H. Therefore, the temperature in the high temperature stack 1 varies in the stacking direction indicated by the arrow L. In other words, in the high temperature stack 1, the reactant gas passages (the fuel gas passage and the oxygen-containing gas passage) have a low temperature on the inlet side, and has a high temperature on the outlet side.
Therefore, the relative humidity in the fuel cells 3a positioned on the outlet side is low, and the overall power generation performance of the high temperature stack 1 is low. In particular, when many fuel cells 3a are stacked in a vehicle application, the power generation performance of the fuel cells 3a positioned on the outlet side is considerably low. The low temperature stack 2 has a similar problem as the high temperature stack 1.